As a method for producing an expansion-molded polypropylene-based resin article that is obtained by molding expanded polypropylene-based resin particles and that has continuous voids (voids in communication with each other), Patent Document 1 has disclosed a method in which a mold is filled with pillar-shaped expanded polyolefin-based resin particles having a specific crystal structure observed by differential scanning calorimetry and having an L/D of 2 to 10, the filling being performed such that the filling rate is 40 to 55% and that the particles are oriented in irregular directions, and then heating is performed with vapor having a temperature within a range from a temperature that is 20° C. lower than the Vicat softening point of a base resin to the melting point.
However, with this method, it is difficult to stably obtain an expansion-molded polypropylene-based resin article with a high porosity, and the porosity is limited to up to 20%. Moreover, achieving a high porosity becomes more difficult as the expansion ratio increases. Thus, this method has poor practicability.
In order to address this problem, Patent Document 2 and Patent Document 3 have disclosed methods for hot-molding expanded polypropylene-based resin particles having a specific shape. The techniques disclosed herein are characterized by using expanded polypropylene-based resin particles that have a hollow cylindrical shape or hollow deformed shapes, or that have a cross-sectional shape with recesses and projections, such as a cross. In order to produce these deformed expanded polypropylene-based resin particles, it is necessary to produce resin particles corresponding thereto. However, production of these resin particles is complicated, resulting in a low productivity of the resin particles having a particle size of 1 to 10 mg/particle, which is the size of expanded polypropylene-based resin particles generally used for foam molding within a mold. Thus, these methods have an economical disadvantage.
Furthermore, in Patent Document 4, the present inventors have disclosed an expansion-molded article having a porosity of 10 to 60%, obtained by using Japanese hand drum-shaped (similar to the shape of an hourglass) expanded particles. According to the technique disclosed herein, the drum-shaped expanded particles are produced from ordinary pillar-shaped resin particles, and then an expansion molded article having a high porosity is produced using the drum-shaped expanded particles. Since common pillar-shaped resin particles are used, this technique does not have an economical disadvantage when producing resin particles. When expanded particles are produced by impregnating resin particles with an expanding agent for a short time, Japanese hand drum-shaped expanded particles are obtained. Herein, a conceivable reason why the expanded particles have a Japanese hand drum-shape is that there is a difference in the amount of the expanding agent impregnated into the resin particles between the end portions and the center portion. Probably due to this factor, a variation in the expansion ratio tends to increase, and thus a variation in the expansion ratio of the expansion-molded article tends to increase.
[Patent Document 1] JP H03 (1991)-224724A
[Patent Document 2] JP H07 (1995)-138399A
[Patent Document 3] JP H07 (1995)-138400A
[Patent Document 4] JP 2000-302909A